Curl and cockle of cellulose-based papers are persistent problems in inkjet printing with water-based inks. The problem stems from dimensional changes in the paper when it is wetted (especially when it is wetted on only one side) and then dried. In normal plain paper, dimensional stability is a function of the presence of cellulose fibers, which are usually a couple of millimeters long. These bind together by fiber-to-fiber associations, which are dominated by intermolecular hydrogen (H) bonds.
When these fiber-to-fiber H-bonds are disrupted or broken, changes in paper physical integrity are brought about. This breaking can be brought about by exposure to elevated temperatures, H-bonding solvents (including water) and/or moisture/humidity.
When aqueous fluid (ink/fixer) is applied to paper, it first accumulates in the paper's capillary spaces. Water and other hydrophilic components of the fluid wet the surfaces of the fibers. This water and/or organic co-solvent breaks the fiber-to-fiber H-bond associations and noticeably reduces the paper's dimensional integrity. With continued exposure of the aqueous-co-solvent fluid to the fibers in the paper, the water and hydrophilic solvents penetrate into the amorphous regions of the cellulose and cause the fibers to swell.
With wetting, the cellulose fiber-to-fiber associations (H-bonds) are disrupted by water and as the fibers swell with water, they increase in size, which relocates the original sites for fiber-to-fiber associations. As the fibers begin to dry from the outside inward, their fiber-to-fiber H-bonds tend to reestablish as surface moisture is lost. As the fibers continue drying out, they shrink from their swollen state, and with the surface fiber-to-fiber associations reestablished, stress/strain develops. This stress/strain is observed as curl across the page.